ensure on_complete called for completed compute log subscriptions
Previously, we were calling on_complete for observers only if the step compute completed
after the observer had subcribed. This change checks for compute completion upon subscription and
calls on_complete accordingly
Test Plan: Opened logs for historical runs, saw complete calls fire.
Reviewers: #ft, alangenfeld
Reviewed By: #ft, alangenfeld
Differential Revision: https://dagster.phacility.com/D1255